Alarm does not sound or only makes a “clicking” noise when tested
Check you are using a high quality alkaline battery (for example Duracell or Energizer). Check battery life.

When the key is turned with the cable end inserted into the lock hole the alarm sounds
Ensure the cable end is clean of dirt or grease and that it is fully inserted all the way into the lock hole. You should also hold the cable in the lock hole at the same time as turning the key. Sometimes, particularly when new, the cable requires gentle force to fit into the lock hole. If the problem still persists, open up the front cover of the lock and check the red insulated wire is connected to the electronic circuit board (as shown on the photo below). If it has become detached, then reconnect the red insulated wire to the circuit board.

Where is the lock hole?
On the front between the lock and the red cable clamp is a small plastic flap. Lift the flap up to reveal the lock hole. You may need to release the red cable clamp first to access the flap.
Difficulties removing the front cover
Firstly ensure the lock cylinder/key is in the unlocked position. With the cable unwrapped locate the two release tabs (as shown in the instruction manual) and using a small screwdriver gently press one of the release tabs whilst easing the cover away from the base. If this is not successful try the same procedure with the other release tab.

How to test the alarm?
Turn the key to the locked position without inserting the end of the cable into the lock hole – the alarm should sound within a few seconds.